Formation of Epitaxial BaTiO3/SrTiO3 Multilayers Grown on Nb-Doped SrTiO3 (001) Substrates.

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The formation of epitaxial BaTiO3/SrTiO3 multilayers is studied in terms of the growth mechanism by investigating surface morphologies, crystalline orientations, microstructures, and structures of the interfaces, as well as by determining the dielectric properties. Under specific conditions, the epitaxial BaTiO3 films follow a layer-then-island (Stranski-Krastanov) mechanism on SrTiO3 (001)-oriented substrates. In view of actual efforts made to grow epitaxial superlattices involving very thin individual layers of BaTiO3 and/or SrTiO3, we have determined that the BaTiO3 films of up to 6 nm thickness do not show any defects and have a sharp BaTiO3-on-SrTiO3 interface. On the contrary, SrTiO3-on-BaTiO3 interfaces within multilayers are rough, probably due to the different growth mechanisms of the two different materials, or due to a difference in the morphological stability of the growth surfaces caused by different surface energies of BaTiO3 and SrTiO3 and by different mobilities of the Ba and Sr atoms reaching the SrTiO3 and BaTiO3 layers, respectively.
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